Dual Use of Traveling and Standing Lamb Waves for Structural Health Monitoring

Structural health monitoring (SHM) is an emerging technology with great potential in structural diagnosis and prognosis. SHM systems can be passive or active. Active SHM systems rely on structural interrogation using active sensors (transducers) that can transmit and receive ultrasonic Lamb waves traveling at large distances in the structural material. Piezoelectric-wafer active sensors (PWAS) are small, inexpensive, non-invasive, transmitters and receivers of ultrasonic Lamb waves that can be permanently affixed to structures. The paper presents results on the dual use of PWAS for structural health monitoring through: (a) tuned traveling Lamb waves using the pitch-catch, pulse echo, and phased-array methods, and (b) broadband standing Lamb wave approach using the high-frequency electromechanical (E/M) impedance method. This paper shows that the traveling-wave and standingwave approaches are complementary to each other and can be used with the same installation of PWAS transducers. Three examples are discussed: (a) detection of cracks in aircraft panel specimens; (b) monitoring of crack growth in an Arcan specimen under mixed-mode fatigue fracture testing; (c) disbond detection in adhesively bonded specimens. In all three cases, both traveling waves and standing waves were used. The paper ends with conclusions and suggestions for further work.